Rabbits have been used to induce specific antisera to several different species of basic protein (BP), including homologous (rabbit) BP. With these antisera we have demonstrated several different specific antigenic regions on the BP molecule. By affinity chromatography on Sepharose to which peptides derived from BP have been coupled, attempts will be made to isolate these specific antibodies. Antisera have also been used for the development of a radioimmunoassay for traces of BP in tissues and body fluids.